A. Viallat - Academia.edu (original) (raw)

Papers by A. Viallat

Revue des Maladies Respiratoires, 2015

Journal de Chimie Physique et de Physico-Chimie Biologique, 1998

The study of conjugated poly(octylthiophene) gels by x-ray diffraction and differential calorimet... more The study of conjugated poly(octylthiophene) gels by x-ray diffraction and differential calorimetry shows that these networks are semi-crystalline. State of crystallinity, transport properties and large scale heterogeneity seem to be correlated. The determination of this correlation requires a detailed study of the mechanisms of crystallization of these polymer gels, the first results of which are reported here. They show that

ABSTRACT The motion and the deformation of soft shells in bounded shear flows is of biological im... more ABSTRACT The motion and the deformation of soft shells in bounded shear flows is of biological importance since, for example, white or red blood cells (RBC) are submitted to strong shear stresses during circulation. The role of cell deformability and viscoelastic properties has not been yet quantitatively studied experimentally although it is important for understanding mobility, binding and rolling of cells on vascular walls. We quantitatively characterized the behaviour of giant lipid vesicles and RBC in a bounded linear shear flow by optical microscopy. The tank-treading motion (fixed cell orientation), the tumbling motion and the lift force experienced by deformable vesicles and RBC close to a substrate, are described as a function of the contrast of viscosity between the inner and the outer fluids, and as a function of the distance from the wall. Results are compared to models developed for ellipsoids of fixed shape. Finally, we showed that RBC oscillate while tank-treading and by increasing the shear rate, they pass from tank treading to tumbling, which discloses the existence of a characteristic time that is related to the visco-elasticity of their cytoskeleton.

Progress in Colloid and Polymer Science, 2006

We prepared giant unilamellar vesicles (GUVs) enclosing solutions or covalent gels of Poly(Nisopr... more We prepared giant unilamellar vesicles (GUVs) enclosing solutions or covalent gels of Poly(Nisopropylacrylamide) (PolyNipam). Concentrated suspensions of GUVs were prepared by applying an alternative field on a lipid film hydrated by a monomer solution containing N-isopropylacrylamide, crosslinker (N,N'methylene-bis-acrylamide), initiator and sucrose. Vesicle inner medium was polymerised and cross-linked by U.V.irradiation of the suspension, yielding viscous vesicles enclosing a solution of linear PolyNipam chains (when no bisacrylamide was used) or elastic vesicles filled with a covalent PolyNipam gel. We show that gel-filled vesicles are responsive systems triggered by the temperature: they shrink, reducing by a factor eight their volume below the critical temperature (32°C in water, lower in presence of sucrose) and re-swell in a reversible and reproducible way upon decreasing temperature. In both cases, we show that the vesicle lipid membrane interacts with the internal polymer, resulting in an strong resistance of the vesicles to external mechanical stresses (enhanced tension of lysis).

Synthetic Metals, 1997

Series of gels covering a wide range of crosslink ratios (from 1/1000 to 1/5) have been obtained ... more Series of gels covering a wide range of crosslink ratios (from 1/1000 to 1/5) have been obtained by connecting poly(3-n-octylthiophene) chains with trithienylbenzenes. We have observed the variation of the swelling ratio, the glass transition temperature and the molecular weight (of the related soluble fractions) of the undoped networks with the crosslink ratio. These quantities exhibit a continuous change. This result is an indication of the homogeneity of the gel structures. Besides, we point out characteristics of the transport properties related to the connectivity.

Physical Review Letters, 2002

Deformation and tank-treading motion of flaccid vesicles in a linear shear flow close to a wall a... more Deformation and tank-treading motion of flaccid vesicles in a linear shear flow close to a wall are quantitatively studied by light microscopy. Velocities of bounded vesicles obey Goldman&amp;#39;s law established for rigid spheres. A progressive tilt and a transition of unbinding of vesicles are evidenced upon increasing the shear rate, gamma;. These observations disclose the existence of a viscous lift force, F(l), depending on the viscosity eta of the fluid, the radius R of the vesicle, its distance h from the substrate, and a monotonous decreasing function f(1-v) of the reduced volume v, in the following manner: F(l) = eta(gamma)(R(3)/h)f(1-v). This relation is valid for vesicles both close to and farther from the substrate.

New Journal of Physics, 2011

We study the sedimentation of buoyant giant lipid vesicles in a quiescent fluid at velocities ran... more We study the sedimentation of buoyant giant lipid vesicles in a quiescent fluid at velocities ranging from 5 to 20 mum s-1. Floppy vesicles are deformed by the flow. Their bottom (upstream) part remains spherical, while their top (downstream) part narrows down and elongates along the direction of motion, resulting in pear-like shapes or in the reversible formation of a

The European Physical Journal E, 2006

The dynamics of giant lipid vesicles under shear flow is experimentally investigated. Consistent ... more The dynamics of giant lipid vesicles under shear flow is experimentally investigated. Consistent with previous theoretical and numerical studies, two flow regimes are identified depending on the viscosity ratio between the interior and the exterior of the vesicle, and its reduced volume or excess surface. At low viscosity ratios, a tank-treading motion of the membrane takes place, the vesicle assuming a constant orientation with respect to the flow direction. At higher viscosity ratios, a tumbling motion is observed in which the whole vesicle rotates with a periodically modulated velocity. When the shear rate increases, this tumbling motion becomes increasingly sensitive to vesicle deformation due to the elongational component of the flow and significant deviations from simpler models are observed. A good characterization of these various flow regimes is essential for the validation of analytical and numerical models, and to relate microscopic dynamics to macroscopic rheology of suspensions of deformable particles, such as blood.

Biophysical Journal, 2005

We report a detailed study of the behavior (shapes, experienced forces, velocities) of giant lipi... more We report a detailed study of the behavior (shapes, experienced forces, velocities) of giant lipid vesicles subjected to a shear flow close to a wall. Vesicle buoyancy, size, and reduced volume were separately varied. We show that vesicles are deformed by the flow and exhibit a tank-treading motion with steady orientation. Their shapes are characterized by two nondimensional parameters: the reduced volume and the ratio of the shear stress with the hydrostatic pressure. We confirm the existence of a force, able to lift away nonspherical buoyant vesicles from the substrate. We give the functional variation and the value of this lift force (up to 150 pN in our experimental conditions) as a function of the relevant physical parameters: vesicle-substrate distance, wall shear rate, viscosity of the solution, vesicle size, and reduced volume. Circulating deformable cells disclosing a nonspherical shape also experience this force of viscous origin, which contributes to take them away from the endothelium and should be taken into account in studies on cell adhesion in flow chambers, where cells membrane and the adhesive substrate are in relative motion. Finally, the kinematics of vesicles along the flow direction can be described in a first approximation with a model of rigid spheres.

Revue des Maladies Respiratoires, 2014